Hon please dont feel bad, your situation sounds like mine..Isla was 4 weeks early, completely sleepy very jaundice and had a very weak suckle reflex. she wouldnt feed from me and i expressed for 4 weeks every 3 hours, she wasnt getting enough to sustain her or get her past the jaundice, when she was almost a month old she still wasnt back to her birthweight so i put her on formula.
That wasnt easy either as she struggled to take a bottle, but after time and after her waking a bit more things got easier!
Just be proud of yourself, LO has got all those antibodies etc, you've done really really well so dont beat yourself up.
Also dont worry about if your LO doesnt take lots of formula either, mine was never a hungry baby...until solids...she loves her grub!